linux-kernel - [PATCH 4/4] UML - Work around host tcsetattr bug

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Message-ID: <20071128190440.GA8313@c2.user-mode-linux.org>
Date:	Wed, 28 Nov 2007 14:04:40 -0500
From:	Jeff Dike <jdike@...toit.com>
To:	Andrew Morton <akpm@...l.org>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	uml-devel <user-mode-linux-devel@...ts.sourceforge.net>
Subject: [PATCH 4/4] UML - Work around host tcsetattr bug

Under the conditions that UML uses it, tcgetattr is guaranteed to
return -EINTR when the console is attached to /dev/ptmx, making
generic_console_write hang because it loops, calling tcgetattr until
it succeeds.  This is a host bug - see
http://marc.info/?l=linux-kernel&m=119618990807182&w=2 for the
details.

This patch works around it by blocking SIGIO while the terminal
attributes are being fiddled.

Signed-off-by: Jeff Dike <jdike@...ux.intel.com>
---
 arch/um/drivers/chan_user.c |   11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

Index: linux-2.6.22/arch/um/drivers/chan_user.c
===================================================================
--- linux-2.6.22.orig/arch/um/drivers/chan_user.c	2007-11-21 11:43:50.000000000 -0500
+++ linux-2.6.22/arch/um/drivers/chan_user.c	2007-11-27 14:03:10.000000000 -0500
@@ -74,10 +74,16 @@ void generic_free(void *data)
 
 int generic_console_write(int fd, const char *buf, int n)
 {
+	sigset_t old, no_sigio;
 	struct termios save, new;
 	int err;
 
 	if (isatty(fd)) {
+		sigemptyset(&no_sigio);
+		sigaddset(&no_sigio, SIGIO);
+		if (sigprocmask(SIG_BLOCK, &no_sigio, &old))
+			goto error;
+
 		CATCH_EINTR(err = tcgetattr(fd, &save));
 		if (err)
 			goto error;
@@ -97,8 +103,11 @@ int generic_console_write(int fd, const 
 	 * Restore raw mode, in any case; we *must* ignore any error apart
 	 * EINTR, except for debug.
 	 */
-	if (isatty(fd))
+	if (isatty(fd)) {
 		CATCH_EINTR(tcsetattr(fd, TCSAFLUSH, &save));
+		sigprocmask(SIG_SETMASK, &old, NULL);
+	}
+
 	return err;
 error:
 	return -errno;
-
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